An air-fuel ratio control device has already been proposed that is equipped with an O2 sensor in which an exhaust gas-side electrode of a sensor element is covered with a catalyst layer, as disclosed, for example, in Japanese Patent Laid-Open No. 11-247687. In the air-fuel ratio control device, the exhaust gas-side electrode is formed on a surface of the sensor element of the O2 sensor, and the exhaust gas-side electrode is covered by a catalyst, layer that removes hydrogen by a catalytic reaction.
In an O2 sensor which does not include a catalyst layer, oxygen (O2) and hydrogen (H2) cause an oxidation reaction at an exhaust gas-side electrode. At this time, since H2 has a higher rate of gas diffusion than O2, H2 is more widely diffused than O2. Consequently, even if the exhaust gas components are in a stoichiometric ratio, a H2-rich atmosphere is generated in the vicinity of the electrode. In such case, since the output of the O2 sensor does not change suddenly unless the components of the exhaust gas become leaner than the stoichiometric ratio, the relationship between the air purification rata and the sensor output shifts to the lean side.
In the above described O2 sensor that includes a catalyst layer, since O2 and H2 react in the catalyst layer and arrive at the exhaust gas-side electrode as an equilibrium gas, it is possible to suppress the occurrence of a situation in which the sensor output shifts to the lean side.